1. Field of the Invention
The invention relates generally to a method for measuring the thickness of a layer and more particularly to a method that correlates changes in input admittance or resonant frequency of a microstrip antenna with changes in the thickness or electrical properties of the layer.
2. Description of the Prior Art
The use, theory and structure of microstrip (patch) antennas is a well-documented field of technology. For example, the book "Microstrip Antenna Design" by K. C. Gupta and Abdelaziz Benalla, Artech House (1988) [hereinafter cited as Gupta and Benalla], is a compilation of approximately fifty-five previously published technical papers on the subject of microstrip antennas.
K. R. Carver and J. W. Mink, in Microstrip Antenna Technology, published in Gupta and Benalla, supra, at 13, disclose the construction of circular, rectangular and two-ear (bunny) patch antennas.
R. J. Mailloux et al, in Microstrip Array Technology, published in Gupta and Benalla, supra, at 36, disclose the use of a coaxial connector, positioned off-center, to ground and excite the patch antenna.
I. J. Bahl et al, in Design of Microstrip Antennas Covered with a Dielectric Layer, published in Gupta and Benalla, supra, at 345, discuss the effect of covering the microstrip antenna with a dielectric layer of material such as Duroid brand fluoropolymer.
V. Hansen et al, in Input Impedance and Mutual Coupling of Rectangular Microstrip Patch Antennas with a Dielectric Cover, published in Gupta and Benalla, supra, at 360, discuss the effect on input impedance of covering the patch antenna with a layer of ice.
The use of a shielded resonant horizontal loop antenna sensor for measuring the thickness of a coal layer is disclosed by Stolarczyk et al, in U.S. Pat. No. 4,753,484, issued June 28, 1988. D. Chang and J. Wait, in "An Analysis of a Resonant Loop as an Electromagnetic Sensor of Coal Seam Thickness," Proceedings of URSI Conference on Remote Sensing, LaBaule, France (Apr. 28-May 6, 1977), developed formulas for calculating the input admittance of a resonant loop antenna for variations in uncut coal layer thickness.
None of the prior art shows the use of a microstrip antenna in a method for measuring the thickness of a layer or for detecting objects concealed by the layer. Furthermore, loop antennas are subject to significant design limitations.